The present invention relates somewhat generally the production of latent photographic images in resist materials and includes the equipment and processes used. Somewhat more particularly, it also relates to methods of semiconductor integrated circuit fabrication which advantageously utilize latent images produced in resist-type materials. It also relates to the design of lithography equipment utilizing latent images.
The following paragraphs include a general discussion of lithography, an explanation of latent imagery, and some discussion of the methods (including latent image measurements) that lithographers use to determine whether their processes are working properly.
Many modem semiconductor fabrication processes involve the deposition of a photosensitive resist material upon a substrate such as a wafer (which may have various material layers formed upon it). The resist material is then exposed to radiation of a particular frequency (or to particles) through a reticle. The radiation interacts with the resist material and produces a pattern which may be considered a three-dimensional distribution of chemical species within the resist. This three-dimensional distribution within the resist is termed a xe2x80x9clatent image.xe2x80x9d Generally speaking, there is desirably a strong correlation between any horizontal (i.e., parallel to the plane of the substrate or wafer) cross-section through the resist material and the image (as spatially filtered by a lens) that the reticle was designed to generate.
In typical semiconductor processing, the resist material containing these latent images may be processed through a number of subsequent steps, and either the exposed or unexposed portion of the resist material is then removed using either wet or dry techniques. (Whether the exposed or unexposed material is removed depends on whether the resist is a xe2x80x9cpositivexe2x80x9d or xe2x80x9cnegativexe2x80x9d resist.) Typical subsequent semiconductor processing often involves steps, such as etching, ion implantation, or chemical modification of the substrate material from which the photoresist has been removed.
The faithfulness of the three-dimensional resist feature (i.e., the latent image formed by the processes mentioned above) in replicating what is desired by the process engineer is controlled by a large number of variables. It is the task of the lithographer to maximize this faithfulness by judicious equipment selection, and adjustment, and definition of usable process windows.
The metrics used by those versed in the art for characterizing the faithfulness of resist feature formation have been the subject of a number of journal articles. In the article, Mack et al., xe2x80x9cUnderstanding Focus Effects In Submicron Optical Lithography, Part 2, Photoresist Defects,xe2x80x9d SPIE Vol. 1088 (1989), the authors discuss the interaction of the aerial image (i.e., the three-dimensional image in free space) with the photoresist and model the interaction mathematically to determine what features of the aerial image are important in determining lithographic performance. In addition, the publication: Mack, xe2x80x9cPhotoresist Process Optimization,xe2x80x9d KTI Microelectronics Seminar Interface 87 (1987), pp. 153-167, discusses various factors which affect the shape of the latent image.
The majority of current practices regarding characterization of faithfulness of the lithographic process to that desired by the process engineer falls into three categories:
i) quantitative measurement of linewidth in etched layers under varying conditions of exposure and focus,
ii) semiquantitative characterization by manual observation (using optical observation or some form of SEM) of features formed under varying conditions,
iii) direct interrogation of the aerial image by scanned slits or image dissecting artifacts.
Some of these will be discussed in more detail.
Practitioners have consistently sought various methods for monitoring the accuracy of the process of transferring the reticle pattern into raised or recessed features on the substrate (i.e., a wafer or various material layers upon it). Since the process involves many steps, it is desirable to analyze and understand each step individually. For example, it is axiomatic that the latent image be as faithful a reproduction of the intended portions of the reticle pattern as possible in order to eventually produce acceptable semiconductor features such as lines, spaces, etc. To evaluate the faithfulness, in the past, practitioners have frequently created latent image lines in photoresist, developed these lines and then etched the line patterns into the underlying substrates of test wafers. Then these test wafers were sectioned and subjected to SEM analysis. The linewidths in the SEM were then measured and compared with the desired linewidth. Unfortunately this monitoring process introduces many additional variables and possible sources of inaccuracy. For example, the developing process, the etching process, and the SEM measurement process may all contribute to inaccuracies in the comparison between the final etched line and the desired reticle xe2x80x9cstandard.xe2x80x9d (Experienced practitioners will also realize that the above procedure may be complicated by etching proximity effects.)
Another technique used to evaluate the adequacy of lithographic processes is to expose the photoresist (to radiation projected from a reticle pattern) thus creating a latent image, and then develop the photoresist. The developed photoresist will exhibit a series of raised features which may be examined and compared with the intended design pattern on the reticle. For example, the developed features may be evaluated by SEM, typically using either high accelerating voltages and conductive coatings on the features or low accelerating voltages coupled with tilted substrate surfaces. The developed features may also be observed and measured optically.
Two factors (characteristic parameters) which are among those that are very important in lithographic processes (and are normally adjusted on a routine basis) are the focus and exposure dose (xe2x80x9cfluencexe2x80x9d) of the stepper or other tool used to produce latent (and then developed) images. Either poor focus or poor exposure dose will create latent images with poorly defined edge profiles and, therefore, ultimately will cause poorly defined edge profiles or unacceptable width variations on etched features or implanted regions. (For example, an exposure dose that is too low provides insufficient energy to induce adequate chemical change in the resist.) Good focus and exposure are often determined experimentally by the following procedure. Either focus or exposure dose is held constant while the other is varied during a series of exposures in which sets of lines and spaces are created in the photoresist Measurements of the linewidth deviation from nominal of either the developed photoresist image or an etched feature are graphically plotted with the variable focus or exposure dose to determine, for example, best focus for a constant exposure dose. The graphical technique often produces curves which, because of their contours, are sometimes called xe2x80x9csmile plots.xe2x80x9d
Recently, one stepper manufacturer has utilized latent imagery in an attempt to determine proper stepper focus. The procedure is as follows: A flat, resist-coated wafer is put in the stepper. A row of approximately 40 local alignment mark features is sequentially exposed in a straight line proceeding horizontally across the entire wafer. The exposure dose is maintained constant while focus is varied for each alignment mark location. Thus a sequence of latent images is created in the photoresist. Each alignment mark consists of several groups of horizontal and vertical lines which are rectangular in shape and which are normally used to provide an xe2x80x9cXxe2x80x9d and a xe2x80x9cYxe2x80x9d (or xe2x80x9chorizontalxe2x80x9d and xe2x80x9cverticalxe2x80x9d) alignment reading. The stepper is then programmed to move the wafer containing the latent images of these alignment marks to a position under its local alignment optics. An alignment is performed at each location and the signal contrast for the X and Y alignments is measured and stored for each alignment mark location. The data is analyzed in an effort to determine proper focus.
It may be noted that in the normal operation of the stepper""s local alignment system the scattered light signal from the edges of a developed or etched alignment mark is measured as the mark is scanned in X and Y coordinates under the system""s illuminating reference beams. (These beams are arranged in shape, spacing and orientation so that they will match the edges of alignment marks.) (The term xe2x80x9cscattered lightxe2x80x9d is used here to indicate light which returns along a path different from that followed by reflected or transmitted light. Because the optics are arranged to observe only the scattered light, the system performs as a dark field microscope and is termed a xe2x80x9cdark field alignment system.xe2x80x9d) However, in the practice described above, the procedure is performed upon a latent image.
After an image scattering signal for the X and Y (i.e., horizontal and vertical) alignment of each alignment mark is measured, a xe2x80x9ccontrastxe2x80x9d number for the X and Y alignment of each alignment mark is determined. The xe2x80x9ccontrastxe2x80x9d is given for the X and Y alignment of each alignment mark as                               K                      i            ,            X            ,            Y                          =                              Imax            -            Imin                    Imax                                    1)            
where Ki,X,Y is the contrast for the ith alignment mark for X or Y, i=1 . . . 40
Imax is the maximum intensity of the scattered signal from the ith alignment mark in the X or Y dimension
Imin is the minimum intensity of the scattered signal from the ith alignment mark in the X or Y dimension.
The X and Y data (i.e., KiX and KiY for each alignment mark on the wafer) are combined (i.e., averaged) to provide an average contrast for each alignment mark. The average contrast for each alignment mark is plotted against mark X-Y location (i.e., the focus used for each mark exposure). The maximum for the latent image alignment contrast curve thus obtained is used as a reference focus to be chosen as xe2x80x9cmachine focusxe2x80x9d for the stepper. xe2x80x9cMachine focusxe2x80x9d is an average reference focus which will be discussed in further detail below.
Applicant has discovered that the latent image based machine focus determination procedure described above suffers from several disadvantages. The format of the previously described technique for determining machine focus requires the stepper to attempt to locally align (in horizontal and vertical directionsxe2x80x94the terms X,Y and horizontal,vertical are used interchangeably herein) to a single row of approximately 40 locations across the face of the wafer and may produce machine focus with somewhat inaccurate results. Should there be small defects on the wafer surface which are less than the focus tracking beam width but of the same or larger size than one or more alignment features within an alignment mark, or if there are irregularities in the stepper chuck, an inadvertent latent image defocus (leading to an inaccurate machine focus determination) will result, or a portion of the latent image may be degraded.
The previously described format is also not generally amenable to choosing proper focus over actual product wafers.
The previously described technique, furthermore, does not provide a method by which one can detect print field curvature.
In addition, the present prior art technique uses a fixed array of alignment marks of relatively constant width occupying approximately 140xcexcm2 area. There is no provision in this technique for varying the linewidth of the alignment marks. Consequently, resolution measurements cannot be made which include the effects due to varying linewidth or neighboring features.
In addition, the previously described technique does not permit one to measure the defocus sensitivity of the lens over various parts of the printing field for various features, geometrics, or orientations.
The present invention addresses one or more of the above problems in an embodiment which includes providing a substrate having a resist; creating a plurality of latent images in the resist, each image being characterized by at least one lithographic parameter, interrogating the latent images and utilizing the results of the interrogation to form additional latent images. The additional latent images may be subsequently developed and the developed images used as a mask to etch, etc., an underlying layer as a part of a semiconductor fabrication process. Examples of lithographic parameters which may characterize a latent image are focus and exposure. Thus, the invention includes a method for choosing proper focus and/or exposure over product wafers.
In another embodiment, the invention includes providing a lithographic tool with a lens and creating latent images in a resist; the latent images are interrogated and the lens is adjusted. For example, in various embodiments, interrogation of the latent images may lead to determination of lens parameters such as astigmatism, defocus sensitivity, depth of focus, resolution, field curvature, spherical aberration, coma, or other aberrations. The the lens system may be adjusted or the stepper adjusted by various means known to those skilled in the art to improve these parameters.
In other embodiments, the interrogation may provide a way of characterizing several lithography tools according to the above parameters. Then the tool with the best resolution, for example, may be selected for certain semiconductor process levels (to the exclusion of other tools).
A few of the advantages of the invention are: it provides a fast method for determining focus and exposure over semiconductor wafer in process at various process levels. The invention also provides a fast quantitative way of determining lens parameters and using them to improve lithography tools. The invention also provides a quantitative way of rank-ordering various steppers so that the xe2x80x9cbestxe2x80x9d may be used at the most critical semiconductor process levels.
A further illustrative embodiment of the invention includes the formation of latent images in full field arrays, which are two-dimensional arrays in a printing field (as opposed to one-dimensional rows in current practice). Illustratively, a full field array contains a matrix of locations in the printing field. Interrogation of latent images at appropriate full field locations is useful in determining lens parameters and lithographic parameters above.
A further illustrative embodiment of the invention includes a method of interrogating latent images which includes forming latent images having at least a first and second dimension. The latent image is exposed to a static energy source which impinges upon the first dimension and not the second dimension. Illustratively, the latent images are rectangles and the energy is a beam which contacts only the lengths of the rectangles. The technique provides latent image data which is free of end defect distortions and noise caused by beam scanning (both of which are problems with currently practiced techniques).